Predicting Critical Period to Characterise Over-Year and Within-Year Reservoir Systems

Surface water reservoirs can beclassified as either within-year or over-year based onthe duration of their critical period (CP). Ingeneral, within-year systems are those which willrefill and spill several times in a year, whereasover-year systems have much longer critical periods,usually of the order of years. If the duration of thecritical period, and hence the precise mode ofbehaviour, of a reservoir were to be known apriori, then advantage could be taken of this toselect the level of detail required for reservoiranalysis. For example, if the reservoir system ispurely over-year, i.e. the CP is much longer than 12months, then only annual streamflow data are requiredfor analysis. On the contrary, systems which exhibitdual within-year and over-year behaviours will requiretime series data of a finer resolution to capture boththe seasonal and annual discrepancies between thedemand and inflow. Such a consideration often resultsin a phenomenal increase in the analysis time overthat required for annual data. Finally, if the systemis purely within-year, then the analysis effort can besignificantly reduced by concentrating on the criticalor driest year of the record. In this paper, weexamine the properties of the test in current use fordistinguishing between within-year and over-yearbehaviours. In particular we investigate how theparameter of the test is related to the CP, and weargue that knowing the CP is a more complete test. Wethen develop predictive equations for the CP and weoffer suggestions for extending the study. Copyright Kluwer Academic Publishers 1999